In the field of positron emission tomography, or PET, it is well known that in order to improve the capability for investigating the living human brain, for example, with regard to blood flow, metabolism and receptor characteristics for small structures such as cortical sublayers and nuclei, the spatial resolution has to be improved relative to what is available today, as disclosed by K. Herholz, et al., "Preoperative activation and intraoperative stimulation of language-related areas in glioma patients," Neurosurgery, 1997; and L. Farde, et al., "A PET study of [.sup.11 C]FLB 457 binding to extrastriatal D2-dopamine receptors in health subjects and antipsychotic drug treated patients," Psychopharmacology, vol. 133, no. 4, 1997, spatial resolution of 2 mm or less may be necessary to reach these research goals. Such a resolution approaches the physical limits of the annihilation process itself; the range of positron in tissue and the non-collinearity of the annihilation photons.
The highest spatial resolution positron camera system commercially available for human investigations presently is the ECAT EXACT HR, as discussed by M. E. Casey, et al., "A multi-crystal two dimensional BGO detector system for positron emission tomography," IEEE Trans. Nucl Sci., vol. 33, pp. 460-463, 1986, with a spatial resolution in the reconstructed image planes of less than 4 mm. The ECAT EXACT HR system uses cost effective block technology and is based on BGO scintillators with 7.times.8 crystals per block with individual crystal sizes of approximately 2.9.times.5.9.times.30 mm.sup.3. By reducing the geometry of the system, the non-collinearity of the annihilation gamma rays are reduced, resulting in a spatial resolution of around 3 mm for a 40 cm diameter system. However, to maintain this resolution over a 20 cm FOV, the DOI in the 30 mm deep crystals must be obtained, which is a challenge with low light yield scintillators, such as BGO or GSO. Also the time response of the used scintillator has to accommodate an excellent timing resolution to suppress random coincidences with a short coincidence window of .about.5 ns. In addition, with the low light from BGO, the 3 mm crystal dimension is probably a practical lower limit which can be resolved with the present BGO based block technology. A cost effective BGO positron camera system based on the block concept with a 2 mm spatial resolution has not been shown to be possible to construct.
Recently a new scintillator has become available, lutetium-oxyorthosilicate (LSO) with a scintillation light yield between four to five times that of BGO and a scintillation decay time of around 40 ns, as disclosed by C. L. Melcher, et al., "Cerium-doped lutetium oxyorthosilicate: A fast, efficient new scintillator," IEEE Trans. Nucl. Sci., vol. 39, no. 4, pp. 502-505, 1992. The high light yield implies that small crystals now can be identified with the block technology with only a small identification degradation due to photon statistics. The short scintillation decay time implies low dead time losses in the detectors. In addition, detectors based on LSO have a good time resolution which offers the possibility of using a short coincidence time window thus reducing the random coincidence contribution. Positron camera systems with a spatial resolution of 2 mm or less are now feasible for investigations of the living human brain as well as for animal studies.
Other related articles include: W. J. Jagust, et al., "The cortical topography of temporal lobe hypometabolism in early Alzheimer's disease", Brain Research, vol. 629, no. 2, pp. 189-198, 1993; C. Carrier, et al., "Design of a high resolution positron emission tomograph using solid state scintillation detectors," IEEE Trans. Nucl. Sci., vol. 35, no. 1, pp. 685-690, 1988; and M. E. Casey, et al., Investigation of LSO crystals for high resolution positron emission tomography," IEEE Trans. Nucl. Sci., vol. 44, no. 3, pp. 1109-1113, 1997. Carrier, et al., disclose a phoswich combination of scintillator detectors for use in a high resolution positron emission tomograph.
An ECAT HRRT, as discussed by Casey, et al., is an octagonal design with 8 detector heads and an axial dimension of 25.2 cm. The distance between two opposing heads is 46.9 cm. Because of the small ring diameter and the large axial and transaxial width, corrections for depth-of-interaction is required to meet the ambitious goal of .about.2 mm spatial resolution. The depth-of-interaction information is extracted from differences in scintillation decay time between the two crystal layers. Casey also disclosed the possibilities to use a LSO-LSO combination of the same dimensions using the differences in scintillation decay time.
An object of the present invention is to provide a new block detector for a new brain camera design, the ECAT HRRT, with a phoswich combination of LSO and GSO crystals with an individual crystal sizes, for example, of around 2.1.times.2.1.times.7.5 mm.sup.3 in each crystal layer.